Improved yttrium and zirconium abundances in metal-poor stars

We present new abundances of the lighter n-capture elements, Yttrium (Z=30) and Zirconium (Z=40) in the very metal-poor, r-process rich stars BD+17 3248 and HD 221170. Very accurate abundances were obtained by use of the new transition probabilities for Y II published by Biemont et al. 2011, and Zr II by Malcheva et al. 2006, and by expanding the number of transitions employed for each element. For example, in BD+17 3248, we find log(Epsilon)=-0.03 +/-0.03 (Sigma=0.15 from 23 lines) for Y II. As for Zr II, log(Epsilon)=0.65 +/- 0.03 (Sigma=0.1 from 13 lines). The resulting abundance ratio is [Y/Zr]=-0.68 +/- 0.05. The results for HD 221170 are in accord with those of BD+17 3248. The quantity of lines used to form the abundance means has increased significantly since the original studies of these stars, resulting in more trustworthy abundances. These observed abundance ratios are in agreement with the r-process only value predicted from stellar models, but is under-abundant compared to an empirical model derived from direct analyses of meteoritic material. This ambiguity should stimulate further nucleosynthetic analysis to explain this abundance ratio.